1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an in-plane switching mode liquid crystal display device with an adjustable viewing angle and a method of fabricating the same. Although the present invention has a wide scope of applications, it is particularly suitable for protecting a personal display device user's privacy and/or security in a crowed public place by selectively operating viewing angles of a personal display device having an in-plane switching mode LCD panel screen.
2. Description of the Related Art
A liquid crystal display device has drawn the most attention among flat display devices. This is because the liquid crystal display device can be operated with a low power and a high definition. It also can be manufactured in a small volume and a large size compared to a cathode ray tube. In general, a liquid crystal display device is operated by changing optical anisotropy through electric fields applied to liquid crystal having both mobility of liquid and optical characteristics of crystal. The liquid crystal display device may be realized in a variety of modes depending on the property of liquid crystal and the structure of a liquid crystal pattern. More specifically, the liquid crystal display device can be operated in a twisted nematic (TN) mode, a multi-domain mode, an optically compensated birefringence (OCB) mode, an in-plane switching (IPS) mode, and a vertical alignment (VA) mode.
In a twisted nematic (TN) mode, directors of liquid crystal are arranged such that they are 90° twisted and are applied by an electric field to control the directors. A multi-domain mode is operated in which one pixel is divided into a plurality of domains to change a direction of the main viewing angle in each domain, to thereby provide a wide viewing angle. In an optically compensated birefringence (OCB) mode, a compensation film is attached to on the outer surface of a substrate to compensate for a phase change in light. In an in-plane switching (IPS) mode, two electrodes are formed on one substrate so that directors of liquid crystal twisted in a plane parallel to an alignment layer. A vertical alignment (VA) mode allows long axes of liquid crystal molecules to be arranged vertically to an alignment layer plane by using negative liquid crystal and a vertical alignment layer.
Among other modes described above, the in-plane switching mode liquid crystal display device includes a color filter substrate (referred to as an upper substrate) and a thin film array substrate (referred to as a lower substrate) disposed to face each other and having a liquid crystal layer between the two substrates. In addition, a black matrix for preventing a light leakage is formed on the upper substrate, and a color filter layer consisting of R, B, and G color resists for realizing colors is formed on the black matrix.
Also, the lower substrate includes gate lines and data lines defining a unit pixel, switching devices formed on intersections between the gate lines and the data lines, and a common electrode and pixel electrodes arranged to alternately cross each other and generating a transverse electrical field.
A related art in-plane switching mode liquid crystal display device will be described with reference to the accompanying drawings.
FIG. 1 is a plan view of an in-plane switching mode liquid crystal display device according to a related art, and FIG. 2 is a cross-sectional view taken along I-I of FIG. 1.
Initially referring to FIG. 1, gate lines 12 and data lines 15 perpendicularly arranged to cross each other on a lower substrate 11, to thereby define pixels, thin film transistors (TFTs) arranged on intersections where the gate lines 12 and the data lines 15 intersect, a common line 25 arranged within each of the pixels to be parallel to the gate lines 12, a plurality of common electrodes 24 branching off from the common line 25 and parallel to the data lines 15, a plurality of pixel electrodes 17 each being connected to each of drain electrodes of the thin film transistors and arranged alternately between the common electrodes 25 in parallel with and with respect to the common electrodes 25, and capacitor electrodes 26 each extending from each of the pixel electrodes 17 and overlapping the upper portion of the common line 25.
Each of the thin film transistors includes a gate electrode 12a branching off from each of the gate lines 12, a gate insulation layer (not shown) formed on the entire surface including the gate electrode 12a, a semiconductor layer 14 formed on the gate insulation layer on the gate electrode 12a, and a source electrode 15a and a drain electrode 15b branching off from each of the data lines 15 and formed at both ends of the semiconductor layer 14.
Each of the pixel electrodes 17 is connected to the drain electrode 15b through a drain contact hole 19. Also, each of the common line 25 and the common electrodes 24 is integrally formed, and simultaneously formed with the gate lines 12. Each of the common line 25 and the common electrodes 24 is formed of low resistance metal such as Cu, Al, Cr, Mo, and Ti.
The pixel electrodes 17 and the common electrodes 24 are alternately formed with respect to each other. The pixel electrodes may be simultaneously formed with the data lines 15 or may be formed of a different layer from the data lines 15. The common electrodes 24 and the pixel electrodes 17 may be alternately formed in a straight line, or may be formed in a zigzag pattern with respect to each other.
The common electrodes 24 and the pixel electrodes 17 may be formed of transparent conductive metal having a desirable light transmittance such as indium-tin-oxide (ITO). A liquid crystal display device having the above-described structure is also called an ITO-ITO electrode IPS liquid crystal display device.
Referring to FIG. 2, an insulation layer is further provided between the common electrodes 24 and the pixel electrodes 17 in order to electrically isolate the common electrodes 24 from the pixel electrodes 17. A reference numeral 13 of FIG. 2 represents a gate insulation layer formed of silicon nitride or silicon oxide.
The common electrodes 24 may be formed first as described above, the pixel electrodes 17 may be formed later, and then portions therebetween may be filled with an insulation layer to electrically isolate common electrodes 24 from the pixel electrodes 17. As an alternative, the pixel electrodes 17 may be formed first, the common electrodes 24 may be formed later, and then the portions therebetween may be filled with an insulation layer to separate common electrodes 24 from the pixel electrodes 17. Also, the common electrodes 24 and the pixel electrodes 17 may be formed of the same layer without an intervening insulation layer. A protective layer 16 for protecting a variety of patterns is further formed on the entire surface including the pixel electrodes 17.
Referring back to FIG. 2, black matrixes 22 preventing a light leakage are provided on an upper substrate (i.e., color filter substrate) 21, and a color filter layer 23 consisting of R, G, and B color resists is provided between the black matrixes. An overcoat layer 29 protecting the color filer layer 23 and planarizing the surface of the color filter layer 23 is provided on the color filter layer 23. The black matrixes 23 extend up to the common electrodes 24 within the pixels located at both ends of the lower substrate 11 to prevent a light leakage from the edge of the pixels.
Also, it is possible to allow the common electrodes 24 located at the edged of the pixels to overlap the data lines 15 and to perform a function of the black matrix 22. In this case, each of the common electrodes 24 should be formed of a light-blocking layer such as a metal layer.
The lower substrate 11 and the color filter substrate 21 of the in-plane switching mode liquid crystal display device are coupled to face each other using a sealant (not shown) having an adhesive characteristic, and a liquid crystal layer 31 is formed between the two substrates as illustrated in FIG. 2.
According to the in-plane switching mode liquid crystal display device having the above-described construction, both the common electrodes 24 and the pixel electrodes 17 are formed on the same substrate in order to rotate liquid crystal molecules 32 while maintaining the liquid crystal molecules parallel to the lower substrate 11. A voltage is applied between the two electrodes to generate a transverse electric field with respect to the lower substrate 11.
This transverse electric field reduces birefringence change of the liquid crystal with respect to a viewing direction. Therefore, the in-plane switching mode liquid crystal display device provides a desirable viewing angle characteristic compared to a twisted nematic mode liquid crystal display device according to the related art.
FIG. 3 is a view illustrating a voltage distribution of a related art in-plane switching mode liquid crystal display device, and FIGS. 4A and 4B are plan views without applying a voltage and with applying a voltage, respectively.
Referring to FIG. 3, when a voltage of 5V is applied to each of the common electrodes 24 and a voltage of 0V is applied to each of the pixel electrodes 17, an equipotential surface is distributed parallel to each of the electrodes 24 and 17 in a portion right above the electrodes 24 and 17, and the equipotential surface is distributed almost vertically in a region located between the two electrodes 24 and 17.
Since the direction of the electric fields is perpendicular to the equipotential surface, a horizontal electric field rather than a vertical electric field is formed in a region located between the common electrode 24 and the pixel electrode 17. Also, a vertical electric field rather than a horizontal electric field is formed in a region located above each of the electrodes 24 and 17, and both the horizontal electric field and the vertical electric field are formed in a composite manner in a region located over the edges of each of the electrodes 24 and 17.
In the in-plane switching mode liquid crystal display device, an arrangement of liquid crystal is controlled by using electric fields. When a sufficient voltage is applied to liquid crystal molecules 32 initially oriented to the same direction as that of a transmittance axis of one of polarization plates as illustrated in FIG. 4A, the long axes of the liquid crystal molecules 32 are oriented parallel to the direction of the electric field as illustrated in FIG. 4B. On the other hand, when the liquid crystal molecules have negative dielectric anisotropy, the short axes of the liquid crystal molecules are oriented parallel to the direction of the electric field.
More specifically, a first polarization plate and a second polarization plate attached on outer surfaces of the lower substrate and the upper substrate to face each other are arranged such that their transmittance axes are perpendicular to each other. A normally black mode is achieved by forming a rubbing direction of an alignment layer on the lower substrate to be parallel with respect to a transmittance axis of one of the polarization plates.
That is, when a voltage is not applied, the liquid crystal molecules 32 are oriented as illustrated in FIG. 4A to display a black state. On the contrary, when a voltage is applied, the liquid crystal molecules 32 are oriented parallel to the direction of the electric field as illustrated in FIG. 4B, to thereby display a white state.
As described above, the related art in-plane switching mode liquid crystal display device has an advantage of having a wide viewing angle. However, there is an occasion where this advantage may cause a problem. For example, when a user uses a personal display device equipped with an IPS mode LCD panel at a public place, his/her privacy and/or security can be invaded by an adjacent person's peek.
To solve this problem, additional liquid crystal panel controlling a viewing angle can be attached on a main liquid crystal panel in order to protect a user's privacy or for a security purpose. The additional liquid crystal panel causes an excessive light leakage in a black state in a horizontal viewing angle direction, so that it can narrow the viewing angle. However, in this case, not only the liquid crystal panel for a viewing angle control should be additionally manufactured, but also the thickness and the weight of a product increase more than twice. Also, when the liquid crystal panel for the viewing angle control and the main liquid crystal panel are attached to each other, a misalign may be occurred. Also, since the light incident from a backlight assembly should further pass through the liquid crystal panel for the viewing angle control when the liquid crystal display device is used in a wide viewing angle mode, front brightness may be considerably reduced.